Roof Area Calculator
Enter the horizontal plan area and slope (%) to compute the actual sloped roof area and slope angle for tiles, waterproofing and roofing material takeoff.
Input Data
Results
At a glance:The actual sloped roof area is larger than the horizontal plan; the relation is actual area = plan area ÷ cos(slope angle). With slope as a percentage s%, the slope angle θ = arctan(s/100) and cos θ = 1/√(1+(s/100)²), so actual area = plan × √(1+(s/100)²). At flat (s=0) actual equals plan; steeper slope means more actual area and more material.
Formula
Slope angle = atan(s/100)
Actual area = plan × √(1+(s/100)²)
$$\theta = \arctan\!\left(\frac{s}{100}\right)$$$$A_a = A_p\sqrt{1+\left(\frac{s}{100}\right)^2}$$How to Use
- Enter the roof horizontal plan area (m²).
- Enter the slope (%).
- The tool outputs the slope angle and actual roof area.
Case Studies
Pitched tile roof
Plan 100 m², slope 30%.
Angle 16.7°; actual about 104.4 m².
Steep metal roof
Plan 80 m², slope 50%.
Angle 26.6°; actual about 94.3 m².
FAQ
Why is actual area larger than plan?
A sloped surface unfolds longer than its horizontal projection; the steeper the slope, the larger the difference, so tiles and waterproofing must be counted on the true slope.
How does this differ from the roof pitch calculator?
The roof-pitch calculator finds the slope angle from rise/run; this calculator finds the actual covering area from plan and slope — different applications.
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References
Content review: Calculatorism Construction Team. Results are for reference only; please refer to the relevant authorities for the official figures.